The present invention relates to moving and transporting stacks of paper and similar materials, and more particularly to the deliberate use of electrostatic charges to pin sheets of paper together in a stack which can then be easily transported and stored.
Electrostatic forces on webs or sheets of paper often interfere with the operation of paper converting machinery. These charges are of opposing polarity so that one side of the continuous web is of positive polarity while the other side is of negative polarity. This causes the web to be attracted to machinery components or to adjacent webs (in case of multiple web processing with webs being unwound from several mill rolls, or when a single web is slit into ribbons which are then superimposed in the processing machine for further processing). Once sheeted, the webs are then stacked onto skids up to about six or seven feet high. The skid loads are then transported by fork-lifts to a temporary storage area or moved directly to the next processing machine such as a destacker and cartonizing machine manufactured and sold by Involvo, 33 Brook Street, West Hartford, Conn.
In order to prevent machine jam-ups due to sheets clinging together from the time they are cut from the endless web until they are stacked at the delivery end of the sheeter, great care is taken to avoid or eliminate electro-static charges between the paper layers. This facilitates the flow of the cut sheets into the delivery stack at the sheeter. As the endless web (ribbons) advances from the unwind roll through pull roller systems before and after the printing stations and through the printing stations, it comes in contact with metal rollers or metal cylinders on one side and rubber or plastic covered rollers or cylinders on the other. Typical examples are: (1) driven steel pull rollers on one side of the web and rubber or plastic covered nipper wheels (or nipper rollers) on the other; (2) rubber covered impression cylinder on one side of the web and a metal gravure cylinder on the other (in gravure presses); and (3) rubber covered blanket cylinder on one side and steel impression cylinder on the other side (in web offset presses). While passing through the nips of such cylinders or rollers of different materials on each side of the web, the electrostatic charges on the paper surfaces are altered and may add to or subtract from the already existing electrostatic charge on that side of the paper and thereby cause an imbalance that interferes with the high speed operation of the paper processing machine.
The prior art teaches the use of static eliminators to neutralize the electrostatic charges on both sides of the webs to reduce or eliminate jam-ups and other interference with the free flow of the still endless paper webs (or ribbons). The simplest method of prior art is the use of metal tinsel connected to ground and with the free ends of the tinsel touching the moving web.
Other prior art methods for eliminating undesired electrostatic charges from moving webs of paper and similar materials include the use of the nuclear static eliminators, air ionizing devices or static eliminator rods. These devices are commercially available and will effectively neutralize the static charges on a moving web or sheet of paper and thus eliminate the undesired clinging together of webs or sheets and their undesired attraction to machinery components or to each other. Eliminating these electrostatic charges causes the sheets to repel each other in a manner similar to that of magnetic poles having like charges. In addition to this, a boundary layer of air remains between several of the uppermost sheets on the stack, such that these sheets slide easily around when the stack is moved such as during transport by fork-lifts. The boundary layer of air is squeezed out between the lower layers when the weight of the sheets accumulated on top exceeds the repelling force from the like charges between the layers of paper plus the force required to move the boundary layer of air. Normal friction then prevents the lower sheets in the stack from sliding, while the upper sheets still are free to slide around.
When the skid loads of paper are moved, the uppermost sheets have a tendency to slide off, fall down to the floor where they are spoiled. This is particularly severe when handling sheets having the so-called "Kromekote" surfaces which are very smooth (very low coefficient of friction) and contain chemicals having a very high dielectric constant (such as the titanates). "Kromekote" sheets (coated on both sides) and having a thickness of 0.008 to 0.010 inches and a sheet size of about 23.times.35 inches sell for approximately one US dollar ($1.00) per sheet wholesale so that the loss of a few sheets from each stack at the paper processing plant could be substantial.
Additionally, when the skid loads of paper stacks are automatically destacked on the INVOLVO destacker, the uppermost sheets of the individual reams being destacked again slide around and cause undesirable trouble until the reams are cartonized. This occurs because the weight has been removed and the charges of like polarity again want to levitate the upper sheets. The faster the machine runs, the more disturbance there is. This then limits the production speed of the machine to well below the rated mechanical speed.